The present invention relates to a room temperature vulcanizable silicone rubber composition and more particularly the present invention relates to a low modulus room temperature vulcanizable silicone rubber composition with a good shelf life which results from the use of certain specific catalyst systems.
Sealants are well known. A sealant and specifically a sealant that is utilized in high rise construction must adhere to the substrate to which it is applied and preferably has elastomeric properties. The use of such sealants in high rise construction is well known in that such sealants are applied where the window pane meets the building so as to seal the glass to the metal frame providing a weathertight seal. In addition and especially in high rise construction there are many areas in which the sealant is desirable for sealing the materials against the elements of weathering so as to prevent moisture and dirt from entering the building once the sealant is used to seal the window pane to the metal frame. Many sealants are used for such an application and specifically polysulfides.
Further, it is especially desirable that the sealant be an elastomer in its cured form, that is, it can be compressed and expanded, that is, it has elasticity and an elastic memory. It is desirable that the sealant be in the form of a true elastomer since it then will expand and contract in the joint in which it is located and, thus, will continue to seal against the elements of weathering.
It should be noted that in some such sealant applications there is preferred a sealant of high tensile strength and good adhesion to the substrate. This is the application where toughness of the sealant is of primary importance and the lack of elasticity of the sealant is not that important. Such sealant applications which are small necessarily apply to the sealing of joints in which there is a small joint movement. Where the sealant is to accommodate fairly large relative expansion and contraction of a joint opening, then it is preferable to utilize a low modulus sealant. A low modulus sealant means a sealant which has a moderate tensile strength, but a high per cent elongation. Such sealants may be made tougher by the incorporation in them of a treated filler. However, the important property in the sealant is its per cent elongation and, thus, it is desirable that the sealant will be able to compress or expand at least 25% of the distance of the thickness of the joint or more preferably be able to expand and compress by 50% of the thickness of the joint. The more the sealant can expand and compress in terms of the thickness of the joint, or of its own thickness, the more desirable it is, as long as the sealant has some adhesion to the substrate, that is, it does not withdraw or release from the substrate when it expands or contracts. One class of such sealants useful in high rise construction are one component room temperature vulcanizable silicone rubber sealants. An example of such a sealant is for instance to be found disclosed in U.S. Pat. No. 3,296,161. This patent discloses the use of a dialkoxydiacyloxysilane additive to improve the bond strengths of a one component room temperature vulcanizable silicone rubber acyloxy system. Another patent which discloses such a sealant is for instance that of Beers U.S. Pat. No. 3,382,205 which discloses a room temperature vulcanizable silicone rubber composition comprising as an additive for improving the adhesion and/or lowering the modulus of a composition, a fluid which is composed of R.sub.2 SiO units R SiO.sub.3/2 units and R.sub.3 SiO.sub.1/2 units where R is a monovalent hydrocarbon radical.
It should be noted that the above are additives to one component room temperature vulcanizable silicone rubber compositions. Such a one part RTV composition (RTV shall be used hereinafter to refer to room temperature vulcanizable silicone rubber compositions) comprise a silanol terminated diorganopolysiloxane polymer where the organo groups are monovalent hydrocarbon radicals, a filler which is selected from reinforcing or extending fillers (which fillers can be treated and untreated-an example of a reinforcing filler being fumed silica) and a cross-linking agent which is preferably methyltriacetoxy silane although it can be any alkyl triacyloxy silane. There is preferably utilized a catalyst with such systems to expedite the cure and such a catalyst is preferably the metal salt of a monocarboxylic acid where the metal varies from lead to manganese in the Periodic Table.
The basic ingredients of the silanol polymer, the filler, the acyloxy crosslinking agent and the metal salt of carboxylic acid are simply mixed under anhydrous conditions. When it is desired to cure the system, the mixture is simply applied and exposed to atmospheric moisture whereupon it cures to a silicone elastomer with a release of acetic acid. In the case where the cross-linking agent is methyltriacetoxy silane, there are many additives that can be added to such a composition to change its properties. The foregoing patents that were disclosed above of Beers '205 Patent and the Kulpa '161 Patent disclosed two additives which may be added to such a system to improve its properties in the case of Kulpa being the adhesion promoter and in the case of the additive of Beers, it being an enhancement of adhesion promotion, as well as lowering the modulus of the system.
In the case of such sealant compositions without major modification of the ingredients, there results a silicone sealant which has plus or minus 25% extension and compression in the joint in which it is placed, of a joint 1/16 to 1 inches wide. Accordingly, it is highly desirable to modify such a traditional sealant of the composition disclosed previously and specifically silicone sealant of the composition disclosed previously, such that it has a plus or minus 50% compression and extension in joints whose width varies from 1/16 to 1 inch.
There are many ways for lowering the modulus of an acyloxy functional silicone sealant, or other silicone sealants for that matter.
One modification that may be made to the silicone sealant is to increase the viscosity or molecular weight or polymer chain length of the base silanol terminated diorganopolysiloxane polymer. It should be noted that the increasing of the viscosity, increasing the molecular weight and increasing the polymer chain length all mean the same thing; that is, by increasing the viscosity, the molecular weight of the polymer is increased which makes the final silicone elastomer more elastic and thus it has a lower modulus. The longer polymer chains will not be as highly cross-linked as shorter polymer chains and as such the silicone elastomer that is formed from the longer polymer chains will be more elastic or have a lower modulus. Another way of decreasing the modulus is to use an extending oil in the composition that is a diorganopolysiloxane polymer which is unreactive to the system and which simply acts as a plasticizer. This again will make the composition more elastic and lower the modulus of the cured system. Another way of lowering the modulus of the system is to add the fluid of the Beers U.S. Pat. No. 3,382,205, such a fluid acts as a chainstopping fluid, thus, lowering the amount of cross-linking that is carried out by the acyloxy functional silane crosslinking agent. The lower crosslinking makes the cured composition more elastic and thus allows it to have a lower modulus.
Finally, fumed silica or precipitated silica that is a reinforcing filler may be added to the composition and preferably such reinforcing filler is treated so as to increase the tensile product of the composition, i.e., the toughness of the composition. Accordingly, all of the above modifications may be made to one component acyloxy functional RTV system, such that it has the plus or minus 50% desired compression and expansion in the joints.
Accordingly, it is highly desirable to formulate an acyloxy functional silicone sealant which has the foregoing low modulus so as to meet the specific compression and expansion requirement set forth above. However, it has been found that when such is done or carried out that the shelf life of the compositions suffer, that is, the composition will have a shelf life of anywhere from 6 months to 9 months, and after that time may cure very slowly or not cure at all. It is also noted that when the sixth or ninth month period is passed, the composition will have a tack-free time that is exceedingly long which is undesirable.
Accordingly, it is one object of the present invention to provide for a low modulus, one package RTV acyloxy functional sealant, which has a good shelf life, that is a shelf life of one year or more.
It is another object of the present invention to provide for a low modulus acyloxy functional one component RTV sealant which has a shelf life of 18 to 27 months.
It is an additional object of the present invention to provide for a composition of a one component acyloxy functional RTV composition which when applied to a joint width of a specified size will have plus or minuss 50% compression and expansion, that is, it is a low modulus silicone sealant which has a short tack-free time and a long shelf life.
It is still an additional object of the present invention to provide for the process of producing a low modulus one component acyloxy functional RTV sealant which will have a short tack-free time and a long shelf life. These and other objects of the present invention are accomplished by means of the disclosure set forth hereinbelow.
In accordance with the above object, there is provided by the present invention a low modulus room temperature vulcanizable silicone rubber composition with a good shelf life comprising (A) a first mixture of (1) 100 parts by weight of a silanol terminated diorganopolysiloxane polymer with a viscosity varying from 50,000-350,000 centipoise at 25.degree. C. where the organic groups are monovalent hydrocarbon radicals; (2) from 0 to 100 parts by weight of a filler; and (B) a second mixture where there is utilized from 1 to 20 parts; by weight of the second mixture per 100 parts of the first mixture of (3) from 60 to 100 parts by weight of an acyloxy functional silane of the formula, EQU RSi(OCOR').sub.3
where R and R' are monovalent hydrocarbon radicals and (4) from 0.1 to 5 parts by weight of a catalyst selected from the class consisting of zinc salts of carboxylic acid, zirconium salts of carboxylic acid and mixtures thereof.
Such compositions while having a good cure and a good shelf life for extended periods of time such as 18 months to 27 months after they are prepared, nevertheless have an extended back-free time, such as 40 to 60 minutes. If it is desired to have a low modulus composition with a good shelf life and a good cure after a period of time of 18 months to 27 months, there is utilized in the mixture (B) in the composition above in addition to the acyloxy functional silane a co-catalyst system. Such a co-catalyst system contains from 0.5 to 5 parts by weight of a tin salt of a carboxylic acid and from 0.001 to 0.4 parts by weight of a co-catalyst selected from the class consisting of a zinc salt of a carboxylic acid and zirconium salt of a carboxylic acid. As pointed out previously, such a co-catalyst system unlike the first catalyst system has a good shelf life, that is the composition will cure after 18-27 months after it has been prepared and will have a short tack-free time of 20 minutes or less; unlike the extended tack-free time of the composition of the first catalyst system. One type of tin salt that can be utilized in the co-catalyst system is dibutyltindilaurate. A much more preferred tin salt which give the advantageous tack-free time in the instant compositions is dimethyl tin neo-decanoate. The preferred zirconium salt is zirconium octoate while the preferred zinc salt is zinc octoate. Other zinc salts and zirconium salts would operate just as effectively in the instant invention. The above compositions may contain any of a number of well-known additives as will be described below.